Recently I was river tracing here in Taiwan. It’s basically hiking but in the water, you just jump into some kind of a stream and go upstream, it is very fun, especially when rocks are slippery because then you always feel like a very bad ballerina. After several hours spent in the water, our fingers became very wrinkly, you know, as they always do after a long bath, a swim or any prolonged water activity. And during the lunch brake people started discussing why does that happen, why do fingers get wrinkly in the water? Most of us started to mumble something about osmosis, i.e. water entering or leaving the skin. Yes, it’s very smart of us to say ‘osmosis’ and pretend that we know something about it but really this was just another case of what episode #293 of This American Life called ‘Modern Jackass’: situations in which people act as if they are knowledgeable about something but they actually have no clue. As Nancy Updike put it, “The thing about Modern Jackass is, it’s usually not something about which you know nothing. It’s something about which you know a little bit, enough to sort of get yourself into trouble.” And to be honest osmosis doesn’t really make sense if you think about it, I mean shouldn’t then all of our body turn wrinkly in water not just the fingers, and at least I haven’t notice any particular differences about wrinkling in fresh and salt water, but clearly the osmotic pressure in the two must be quite different (it’s easy for me to speak post factum).

Fig.1 Wrinkly Fingers

I kind of forgot about this wrinkly business afterwards but then one day I was looking through Reddit and saw one trending post suggesting that a study found why fingers wrinkle. Obviously this reminded me about our modern jackass talk and so I went on to read the actual study. Spoiler alert: we still actually don’t know the answer (figure 2 is misleading), but the story is at least an example of a self-correcting scientific process.

Fig.2 Misleading post

Initially, it was indeed considered that the wrinkling is caused by osmosis as a result of water entering the outermost layer of the skin and swelling it. Interestingly, however, doctors have noted a while ago that the fingers of some people with certain nerve damage actually fail to wrinkle after immersion into water, which suggests that the nervous system must be involved. Later studies have showed that the blood vessels in the fingers, controlled by the body’s autonomous nervous system, constrict which in turn leads to finger wrinkling. Water induced finger wrinkling is actually now used as clinical indicator for functional autonomous nervous system.

Recently, however, a hypothesis has been proposed which suggests that the wrinkling might be an evolutionary adaptation to make the handling of objects underwater easier. Wrinkling creates a kind of drainage path for water and so enhances the grip on an object (this is called a ‘rain tread’ hypothesis). In order to test if this hypothesis is true Kareklas et al. have recruited volunteers and tested their ability to transfer wet objects when the fingers are wrinkled and not. The test went as follows: 20 participants had to transfer glass marbles from one container to another in two different conditions (1) take the marble from a container with water pass it through a small hole and put into an empty container and (2) take the marble from a container without water pass it through a small hole and put into an empty container. For each participant scientists measured the time it took to achieve the task with wrinkly and smooth fingers.

Fig.3 Transfer time (standardized to the time taken to transfer dry objects with unwrinkled fingers) is shortest for dry objects, independent of wrinkling, but faster for submerged objects with wrinkled (red bar) fingers than with unwrinkled (black bar) fingers (***p < 0.001). Items are picked up with thumb and index finger of the right hand, passed through a hole to the left hand, and put into a box with a hole in the lid.

The results showed that it takes longer to transfer object that is wet. If the marble ball was dry there was no difference between the transfer time with wrinkly and smooth fingers, however, if the marble was wet then on average it took 12% less time to transfer the object with wrinkly fingers. Therefore, the study concluded that the wrinkling of fingers improves the handling of wet objects (which supports the rain tread hypothesis). However, why are our fingers not always wrinkled then? If wet object handling is made easier and there is no difference with dry objects one would think it would be beneficial to always have the wrinkles. In paper’s discussion Kareklas et al. suggest that there potentially are some fitness trade-offs to the wrinkly fingers. Maybe wrinkled fingers are less sensitive to pain, pressure, heat etc. and are therefore damaged easier, which would explain why it is not good to always have those wrinkles.

At this point I was very happy knowing that the next time this topic is discussed I would be able to say something ‘smart’ about it. But as Kareklas et al. study was published in 2013 so I wanted to google the subject in case someone has done any follow-up studies. And low and behold a study from 2014 comes up entitled ‘Water-Induced Finger Wrinkles Do Not Affect Touch Acuity or Dexterity in Handling Wet Objects’. This study, published in Plos One, in fact initially wanted to take off where the Kareklas et al. study has finished, i.e. to see if skin wrinkling reduces its sensitivity. To do that scientists have designed tests to measure the sensitivity of mechanoreceptors in fingers. One test, called the tactile acuity tests, required participants to determine the spacing of gratings on a side of a cube when it is placed on their finger, and another test measured the highest amplitude of vibrations that a participant can detect. Both tests were done with water-induced wrinkled and smooth fingers. Surprisingly, the tests showed that the sensitivity of mechanoreceptors did not change regardless of finger wrinkling.

Given the negative results, scientists then decided to repeat exactly the same experiment as Kareklas et al. did with their own volunteer group. The repeat this time provided no evidence for improved wet object handling with wrinkly fingers. Although this does not mean that the previous study was wrong, it suggest that the experimental design used is probably not sensitive enough to reproducibly prove the hypothesis. The authors from the Plos One study suggest that maybe a better way would be to measure and compare the forces required to pull out an object held between wrinkly and not wrinkly fingers. I’ll be on a lookout for that paper!

In the end we still don’t know what is the purpose of water induced wrinkling of fingers. But at least I now know its not osmosis and while it potentially could be just a byproduct of blood vessel constriction maybe future studies can prove it has an evolutionary benefit as well.